This invention relates to fastening devices generally and more particularly to devices designed to accurately and conveniently indicate a predetermined tension in a loaded joint.
It has been found desirable in many fastener applications to know that the tension produced in a joint is of a predetermined value and to ascertain when the joint has, in some manner, relieved the tension.
Frequent attempts in the prior art to identify and indicate the tension in the joint utilize the measurement of a more readily definable parameter, such as torque, used to apply the fastener. Since the actual tension produced in the joint is only roughly proportional to the torque, these devices have been found to be somewhat unreliable and inaccurate.
It has been found that the tension in a joint can be identified through the use of a preassembled fastener unit which includes a rotary fastener member having a conical-like washer preassembled thereto and which further includes a 1974, gauge preassembled to these two elements and interposed between the bearing surface of the fastener member and the conical washer. Such a device will properly indicate the tension in a joint based upon the relationship of the compression of the washer to a load applied to it. When a predetermined load, as identified by a certain compression of the washer, is reached the gauge will be pinched between the washer and the bearing surface thus serving to indicate the attainment of such a tension level. Of course, once the joint has relaxed, the gauge member will no longer be pinched in this manner and thus a policing of the joint is obtained through such a device. Exemplary of such a device is the tension indicating unit shown in commonly assigned application, Ser. No. 437,597, filed on Jan. 28, 1974, now U.S. Pat. No. 3,881,392.
While a device of the type just described provides an adequate indication of attainment of broad tension levels in a joint, it is often desirable to indicate tension levels in a joint with a very high degree of accuracy and within a very narrow range of loads on the joint.
Accordingly, it is an object of this invention to provide a preassembled fastener unit of the type including a rotatable fastener member, a conical-like spring washer and a gauge member adapted to be pinched therebetween which is capable of accurate indication of tension in a joint.
It is a further object of the invention to provide a preassembled fastener device capable of minimizing the effects of conventional manufacturing tolerances and discrepancies in a device of the type described.
It is still a further object of the invention to provide a preassembled fastener device and method of calibrating the same wherein the deflection or compression of the washer may be directly and predictably relatable to the load applied thereto.
It is thus an advantage of the present invention that a tension indicating fastener device, capable of accurate measurements of tension level in a joint, can be produced without the requirement that manufacturing tolerances in the production of the individual elements be reduced.
The above and other objects and advantages are obtained by the present invention wherein a fastener assembly including a rotatable fastener member, a generally conical spring washer preassembled thereto, a ring gauge positioned between the washer and the fastener member provided with compressible means permitting the ring gauge to be compressed relative to the fastener member and spring washer during a preloading thereof. Such compressible means may take the form of protuberances, increased thickness of material associated with interruptions in the inner periphery of the gauge or other means capable of permitting a reduction in the transverse dimension of the ring gauge to become calibrated so as to be subsequently capable of indicating a predetermined tension level in the fastening device. A method of calibrating such a device is also disclosed and claimed wherein the preassembled unit with said compressible means is subjected to a compression load to deflect the spring washer to a flattened or other predetermined position so that subsequent compression of the washer device will be in accordance with a reliable load-deflection curve. This aspect of the invention in conjunction with the calibration of the ring gauge to an accurate dimension provides a fastener device which is thereafter capable of accurately indicating tension levels in the system.